J. Polan

Astrophysical radiative shocks: From modeling to laboratory experiments

Radiative shock waves are observed around astronomical objects in a wide variety of environments, for example, they herald the birth of stars and sometimes their death. Such shocks can also be created in the laboratory, for example, by using energetic lasers. In the astronomical case, each observation is unique and almost fixed in time, while shocks produced in the laboratory and by numerical simulations can be reproduced, and investigated in greater detail. The combined study of experimental and computational results, as presented here, becomes a unique and powerful probe to understanding radiative shock physics. Here we show the first experiment on radiative shock performed at the PALS laser facility.The shock is driven by a piston made from plastic and gold in a cell filled with xenon at 0.2 bar. During the first 40 ns of the experiment, we have traced the radiative precursor velocity, that is showing a strong decrease at that stage.Three-dimensional ~3D! numerical simulations, including state-of-art opacities, seem to indicate that the slowing down of the precursor is consistent with a radiative loss, induced by a transmission coefficient of about 60% at the walls of the cell. We infer that such 3D radiative effects are governed by the lateral extension of the shock wave, by the value of the opacity, and by the reflection on the walls. Further investigations will be required to quantify the relative importance of each component on the shock properties.

Experimental study of radiative shocks at PALS facility

We report on the investigation of strong radiative shocks generated with the high energy, sub-nanosecond iodine laser at PALS. These shock waves are characterized by a developed radiative precursor and their dynamics is analyzed over long time scales (50 ns), approaching a quasi-stationary limit. We present the first preliminary results on the rear side XUV spectroscopy. These studies are relevant to the understanding of the spectroscopic signatures of accretion shocks in Classical T Tauri Stars.

Single-shot soft x-ray laser-induced ablative microstructuring of organic polymer with demagnifying projection

We report on a single-shot micropatterning of an organic polymer achieved by ablation with demagnifying projection using a Ne-like Zn 21.2 nm soft x-ray laser. A nickel mesh with a period of 100 microm was approximately 10x demagnified and imprinted on poly(methyl methacrylate) via direct ablation. The quality of the ablated microstructure was found to be mainly dependent on the quality of the projected mask. This first demonstration (to our knowledge) of single-shot projection, single-step lithography illustrates the potential of soft x-ray lasers for the direct patterning of materials with a resolution scalable down to submicrometer domain.

25TW Ti:sapphire laser chain at PALS

We present an overview of the Ti:sapphire laser chain recently commissioned at the PALS laboratory. The laser is based on commercial laser units and of an in-house-designed-and-built compressor. The system provides peak power of 25 TW in <40-fs pulses and delivers up to 0.9 J on the target in the main beam, at a repetition rate of 10 Hz. The laser chain employs conventional CPA amplification technique consisting of oscillator, stretcher, regenerative amplifier, pulse picker, and multipass amplifier, followed by compressor. The compressor is designed to use residual zero diffraction orders to produce two additional 50-mJ beams. One of these beams is compressed by an additional small-size compressor. All three beams can be delayed with respect to each other in the range of about 0-20 ns. The beams are delivered by vacuum distribution system into a target room serving to development of sources of X-rays and accelerated particles. In the near future the system will be synchronized with the PALS kJ laser and will serve as an ultrafast diagnostic probe beam.

Double Lloyd’s mirror: versatile instrument for XUV surface interferometry and interferometric microscopy

We have developed a double Lloyds mirror wavefront-splitting interferometer, constituting a compact device for surface probing in the XUV and soft X-ray spectral domain. The device consists of two independently adjustable superpolished flat surfaces, operated under grazing incidence angle to reflect a diverging or parallel beam. When the mirrors are appropriately inclined to each other, the structure produces interference fringes at the required distance and with tuneable fringe period. The double Lloyds mirror may be used alone for surface topography with nanometric altitude resolution, or in conjunction with an imaging element for interferometric XUV surface microscopy. In the latter case, resolution in the plane of the probed surface is about micron, which is given by the quality of the imaging element and/or by the detector pixel size. Here, we present results obtained using the double Lloyds mirror in two separate X-ray laser and high harmonics generation (HHG) application projects. The first experiment was aimed at understanding microscopic nature of the effects involved in laserinduced optical damage of thin pellicles, exposed to sub-ns laser pulses (438 nm) producing fluence of up to 10 Jcm-2. The probing source in this case was a QSS neon-like zinc soft X-ray laser, proving a few mJ at 21.2 nm in ~100-ps pulses. The second experiment was carried out using a narrowly collimated HHG beam near 30 nm, employed to topographically probe the surface of a semiconductor chip.

Ablative microstructuring with plasma-based XUV lasers and efficient processing of materials by dual action of XUV/NIR–VIS ultrashort pulses

We report on a single-shot micropatterning of an organic polymer achieved by ablation with demagnifying projection using a plasma-based extreme ultraviolet (XUV) laser at 21 nm. A nickel mesh with a period of 100 μ m was 10×demagnified and imprinted on poly(methyl methacrylate) (PMMA) via direct ablation. This first demonstration of single-shot projection, single-step lithography illustrates the great potential of XUV lasers for the direct patterning of materials with a resolution scalable down to the submicrometer domain. In the second part, we present a novel experimental method for improving the efficiency of surface processing of various solids achieved by simultaneous action of XUV, obtained from high-order harmonic generation, and near-infrared (NIR)–VIS laser pulses. The NIR–VIS pulse interacts with free charge carriers produced by the energetic XUV photons, so that its absorption dramatically increases. Laser-induced periodic surface structures were effectively produced using this technique.

Characterization of focused beam of desktop 10-Hz capillary-discharge 46.9-nm laser

The desktop capillary-discharge Ne-like Ar laser (CDL) providing 10-μJ nanosecond pulses of coherent 46.9-nm radiation with a repetition rate up to 12 Hz was developed and built at the Colorado State University in Fort Collins and then installed in Prague. The beam of the laser was focused by a spherical mirror covered with Si/Sc multilayer coating onto the surface of poly(methyl methacrylate) - PMMA. Interaction parameters vary by changing the distance between sample surface and beam focus. The samples were exposed to various numbers of shots. Analysis of damaged PMMA by atomic force (AFM) and Nomarski (DIC - differential interference contrast) microscopes allows not only to determine the key characteristics of the focused beam (e.g. Rayleighs parameter, focal spot diameter, tight focus position, etc.) but also to investigate mechanisms of the radiation-induced erosion processes.

Development of Plasma X-Ray Amplifiers Based on Solid Targets for the Injector-Amplifier Scheme

Results of experimental studies aimed at generation and diagnostics of advanced soft X-ray amplifiers, produced from solid targets, are presented. 2D profiles of electron density of short plasma columns, generated by ~300-ps laser pulses under various illuminating conditions, were investigated by near-field distribution of the plasma self-emission, and by X-ray laser backlighting at 21 nm, accessing in the given geometry electron densities of 1022 cm-3. The obtained data indicate that by employing line focus with concave intensity profile it is possible to generate laterally highly uniform plasma columns of width ~500 °m, potentially suitable as amplifiers with negligible lateral refraction. By X-ray laser backlighting we further probed the morphology and gain region of test Zn plasmas, pumped by a sequence of a loosely focused weak prepulse and tightly focused main pulse, separated by 5.5 ns. The data clearly show the beneficial role of the prepulse in lateral homogenization of the plasma, and reveal narrow ~50-°m gain region.

Development and applications of multimillijoule soft X-ray lasers

We review development of multimillijoule X-ray lasers and of applications of these new laboratory sources carried out recently at the PALS facility. A backbone of this development is the neon-like zinc laser providing saturated output at 21.2 nm, with up to 10 mJ of energy per pulse. This represents currently the most energetic soft X-ray laboratory source. Recent improvements in its operation include better control of the beam shape, and more complete understanding of the prepulse pumping. The laser at 21.2 nm has been employed for a number of application experiments reviewed in this paper. They include transmission measurements of intense soft X-ray radiation, studies of fundamental processes of soft X-ray ablation, ablation micropatterning, feasibility study of soft X-ray Thomson scattering from dense plasmas, visualization of nanometric transient perturbation of optical surfaces, measurements of ablation rates of foils heated by IR pulses, and studies of 2D plasma hydrodynamics in the regime of sequential illumination.

Material ablation induced by focused 21.2-nm radiation from Ne-like Zn x-ray laser

Radiation from the Ne-like Zn soft x-ray laser (λ=21.2 nm, τ< 100 ps) driven by PALS (Prague Asterix Laser System) was successfully focused with a spherical Si/Mo multilayer-coated mirror to ablate poly(methyl methacrylate), monocrystalline silicon, and amorphous carbon. To our knowledge, this was the first observation of material ablation with a laser working in the soft x-ray region, i.e. λ<30 nm.